1. Field
The present invention relates generally to wireless communications, and more specifically to synchronizing the timing of wireless communication devices.
2. Background
In today's electronically interconnected world, the normal complement of electronic equipment in the home or business includes devices that are connected to one another in different ways. For example, many desktop computer systems have a central processing unit (CPU) connected to a mouse, a keyboard, a printer and so on. A personal digital assistant (PDA) will normally connect to the computer with a cable and a docking cradle. A television may be connected to a VCR and a cable box, with a remote control for all three components. A cordless phone connects to its base unit with radio waves, and it may have a headset that connects to the phone with a wire. In a stereo system, the CD player, tape player and record player connect to the receiver, which connects to the speakers. These connections can be difficult to install and maintain, particularly for the lay user.
Alternatives to these conventional approaches to connectivity have been proposed. Bluetooth™ (BT) is a computing and telecommunications industry specification for connectivity that is both wireless and automatic, as described in The Specification of the Bluetooth System, Version 1.1, Feb. 22, 2001, (“the BT specification”), which is incorporated herein by reference. BT allows any sort of electronic equipment—from computers and cell phones to keyboards and headphones—to make its own connections, without wires, cables or any direct action from a user. Because BT connections are wireless, offices can be designed without regard to cable placement and users can travel with portable devices without having to worry about carrying a multitude of cables. These connections can be established automatically, where BT devices find one another and form a connection without any user input at all.
BT requires that a low-cost microchip transceiver be included in each device. The BT microchip transceiver communicates on a frequency of 2.45 GHz, which has been set aside by international agreement for the use of industrial, scientific and medical devices (ISM). In addition to data, up to three voice channels are available. Each BT device has a unique 48-bit device address from the Institute of Electrical and Electronics Engineers 802 standard. Connections can be point-to-point or multi-point. Data can be exchanged at a rate of 1 megabit per second (up to 2 Mbps in the second generation of the technology).
A number of common consumer devices also take advantage of the same RF band. Baby monitors, garage-door openers and some cordless phones all make use of frequencies in the ISM band. The BT design employs various techniques to reduce interference between these devices and BT transmissions. For example, BT avoids interfering with other systems by sending out relatively weak signals of 1 milliwatt. By comparison, some cell phones can transmit a signal of 3 watts. The low power limits the range of a BT device to about 10 meters, thereby reducing the probability of interference with other devices.
BT also employs a spread-spectrum frequency hopping scheme to further reduce interference and increase capacity. BT devices use 79 randomly chosen frequencies within a designated range, changing from one to another on a regular basis 1,600 times every second. The random frequency hopping pattern makes it unlikely that two BT transmitters will be on the same frequency at the same time, thus reducing the probably of BT devices interfering with one another. This technique also minimizes the risk that other non-BT devices such as portable phones or baby monitors will disrupt BT devices since any interference on a particular frequency will last only a fraction of a second.
When BT devices come within range of one another, an electronic conversation takes place to determine whether they have data to share or whether one needs to control the other. Once the conversation has occurred, the devices form a Personal-Area Network (PAN) or “piconet”. A piconet may link devices located throughout a room, such as a home entertainment system, or devices much closer together such as a mobile phone on a belt-clip and a headset, or a computer, mouse, and printer. Once a piconet is established, the connected devices randomly hop frequencies in unison to communicate with one another and avoid other piconets that may be operating nearby.
In the piconet configuration, the connected devices act as either masters or slaves, and one master device may control multiple slaves, and, indeed, a master device may, itself, be a slave to another master device.
This master-slave configuration requires that the slave react subserviently to its master device, and one way that this occurs is through establishing appropriate timing. Specifically, a slave device must synchronize its timing with that of its master. Thus, if two slaves, for example, are in communication with the same master device, they both synchronize their timing with that of the master device.
If, however, a slave moves from one master device to another (referred to hereinafter as “handoff”), the slave device must synchronize its timing with the timing of its new master device. Since the slave has been communicating with the old master using the old master's timing, and is familiar only with this timing, the new master device must “speak” to the slave in the old master's timing. The new master device does this to notify the slave of the new master's timing information so that the slave may synchronize to it. In order for this to happen, the new master device needs to obtain the old master device's timing. The BT Specification, however, does not specify a procedure for providing the timing from one master device to another to effectuate an efficient handoff.
There is therefore a need for an improved system and method for providing timing information to master devices so that handoffs can be accomplished efficiently.